Nobody actually knows what your treadmill pace is worth outside

The one percent incline convention got attached to a single narrow study and then repeated until it became a fact.

Set the treadmill to one percent and it is the same as running outdoors. Everyone repeats this. It came from a limited piece of work looking at a particular speed range in a particular setting, and it has since been applied to every runner at every pace as though it were a physical constant.

The physics behind the correction is real enough. Running outdoors you push through air, and the cost of that rises steeply with speed. On a treadmill the air is not moving relative to you, so that cost disappears, and adding a slight incline substitutes some vertical work for the missing air resistance. Reasonable idea.

The problem is that air resistance depends on how fast you are going, and gradient cost depends on how heavy you are and how efficiently you climb. Those two things do not scale together. At a slow jogging pace the air cost is small and one percent is probably an overcorrection. At genuine race speeds the air cost is much larger and one percent is not enough. A single fixed number cannot be right at both ends, and it is a strange thing to have accepted for so long.

Then there is everything the correction ignores. The belt is driven, so the demand on the hamstrings during the pull through phase differs from overground running. Belt speed accuracy on a lot of machines is poor and drifts with load, wear and calibration, and gyms rarely check it. The room is warm and still, so heat builds up and heart rate climbs for reasons unrelated to mechanical work. Stride tends to shorten because people run nearer the front of the belt than they think. Some runners hold the rail without noticing.

Which is why I would stop converting altogether. Treadmill running is a training modality, not a measurement of outdoor pace, and the effort to make it commensurable produces false precision that flatters or punishes people at random.

Use effort. Use heart rate on a machine you use regularly. Compare treadmill sessions to your own previous treadmill sessions on the same machine, which is the only honest comparison available, and stop trying to work out what the numbers would be if you were outside.

If you want to know what you can run outside, go outside.